Program: GN-2019B-FT-102

The Zwicky Transient Facility (ZTF) is a new large area synoptic survey using the 48 inch Samuel Oschin Schmidt telescope at Mount Palomar. As part of ZTFs operations we conduct a high-cadence Galactic Plane survey with the primary scientific goal to discover and determine the space density of ultracompact binary systems. As part of this survey we discovered ZTFJ2055+4651 which stood out due to its short orbital period of only 56min, its absolute magnitude consistent with a compact helium sar (sdB) and its remarkable lightcurve shape, conisistent with a Roche Lobe filling star being eclipsed by an accretion disc. We obtained a sequence of Shane telescope spectra taken consecutively which show indeed an sdB star and a radial velocity shift of 600 km/s over a 25min time interval. We therefore believe that ZTFJ2055+4651 is the first binary system with a white dwarf accreting from a compact sdB donor star in an ultracompact orbit. Here we request phase resolved spectroscopy over the binary orbit which will allow us in combination with the lightcurves to determine precise system parameter such as mass ratio, component masses and inclination angle. Given the large radial velocity shift the hidden stellar component is likely to be massive makeing the system a potential supernova Ia progenitor.